Mechanical properties of tissue have important use in medical applications because these properties are closely linked to tissue state with respect to pathology. For example, measurement of liver stiffness can be used as a non-invasive alternative for liver fibrosis staging. Recently, ultrasound imaging was developed to measure mechanical properties of tissues. Typically, an ultrasound transducer fires a long duration, focused ultrasound pulse to a displacement origin to induce a displacement motion within a tissue of interest. As a result of this displacement motion, a shear wave propagates outwards from the displacement origin. Tissue mechanical properties can be determined by detecting this shear wave. For example, a time to peak displacement for two or more sample positions with known distance along the propagation path is detected to measure a shear wave velocity. Using the shear wave velocity, tissue mechanical properties can be calculated.
To detect the peak displacement, a temporal displacement for each sample position is measured by repeatedly transmitting ultrasound pulses to and receiving echo signals from the sample position. In conventional peak displacement detection, pulse repetition frequency (PRF) of the ultrasound pulses defines a sampling rate of the temporal displacement measurement and directly affects the accuracy of the peak displacement detection. As transmission of a new ultrasound pulse has to occur after echoes in response to the previous ultrasound pulse have been received from the depth of interest, the PRF cannot be readily increased to improve the sampling rate. The limited sampling rate impedes conventional ultrasound systems to perform peak displacement detection accurately.